Physiological investigations of glia-neurone interactions will be continued using the retina of the honeybee drone as a model preparation. This preparation has useful features that make possible experiments that have not yet been performed on mammalian preparations. These features include: (1) The retina is composed of essentially one type of neurone, the photoreceptor cells, which can be stimulated by light, and one type of glial cell. (2) Activities of intracellular ions, particularly K+ and Na+, can be measured with ion-sensitive microelectrodes, in both photo-receptors and glial cells. (3) Ionic and metabolic interactions between glial cells and photoreceptors have been demonstrated. (4) The kinetics of oxygen consumption by the photoreceptors, after a singe flash of light, can be measured. Investigation of the mechanisms by which K+ enters the glial cells when the photoreceptors are stimulated will be continued. Measurements will be made with intracellular Cl- sensitive microelectrodes, and a quantitative synthesis will be attempted of the measured changes in K+, Na+ and Cl- in the photoreceptors, glial cells and extracellular space. The possibility that some of the intracellular Na+, K+ and Cl- is 'bound' and not detected by ion-sensitive electrodes will be investigated by measuring total concentrations of these elements with an electron beam microprobe. To measure the volume of the extra-cellular space a new method will be used in which the diffusion of extra-cellular probe ions through a slice of retina will be measured with ionsensitive electrodes outside the tissue. This method will also permit an investigation of possible interactions between fixed extracellular charges and mobile ions. Control of mitochondrial respiration in functioning tissue will be investigated by looking for an increase in ATP concentration after a brief stimulus.